The present invention is related to an improved method for forming electronic components. More specifically, the present invention is related to a sintering method wherein materials which require high sintering temperatures can be manufactured with an integral material which is not compatible with high temperature sintering thereby allowing the formation of capacitive structures which were previously unavailable and providing functionalized capacitors without compromising the capacitance.
Capacitors are well known in the art of electronic circuitry and they find widespread use in various applications as widely known and practiced in the art. In general, a capacitor comprises at least two conductors with a dielectric between adjacent conductors. Metals are the preferred conductor, due to their low resistance, and ceramics are the preferred dielectric due to their high dielectric constants. Unfortunately, ceramics must be fired at high temperature to achieve adequate density and these temperatures are often detrimental to the metal. In the case of base metals a neutral or reducing atmosphere is required that adds further restraint on developing compatible ceramics.
Electrolytic capacitors typically use high melting point conductive metals such as tantalum, niobium or aluminum, as the conductive anode or even metal oxides such as niobium oxide. Thin oxide layers are formed on these as the dielectric but since they are very thin the voltage handling capability of the resulting capacitors is limited.
Metalized polymeric films are also used to form capacitors. In these cases the volumetric efficiency of the capacitors are limited by the low dielectric constant of the polymer. Furthermore most polymer film capacitors are not suitable for use at higher temperatures due to their low melting point. Neither film nor electrolytic capacitor technologies can be easily combined with high dielectric constant ceramics since they do not allow for sintering of the ceramic. As a result the applications for these types of capacitor are limited.
Another type of capacitor widely used is multi-layered ceramic capacitors which are well known in the art and used through the electronics industry. There has been an ongoing desire to incorporate additional functionality into multi-layered ceramic capacitor components, such as resistance, inductance, fuses, and the like, to allow for further miniaturization of electronic circuits. The high temperature sintering requirements of the ceramics in multi-layered ceramic capacitors has made efforts to combine functionality very difficult.
Multi-layer ceramic capacitors (MLCC) are formed by interleaving thin layers of ceramic insulator electrodes of opposed polarity and co-sintering to produce a monolithic component. The layering process requires the ceramic and electrode to be suspended in organic media. In general, the ceramic and metal layers are alternately cast, by one of a myriad of techniques, to form a monolith which is first heated to remove volatiles and then sintered as a monolith. The sintering process requires a high firing temperature, typically >800° C. Air atmospheres can be used in the case of noble, or precious, metals such as palladium, silver, gold or their alloys. For base metals such as nickel or copper a reducing atmosphere, typically nitrogen/hydrogen is required to prevent oxidation of the metal and the sintering is followed by an annealing at lower temperature to replace the oxygen vacancies formed in the dielectric which returns the ceramic to an insulating state. The ceramic and electrode materials must be carefully matched during the layering and thermal processing to avoid stresses and subsequent flaws, such as delamination, that compromises the reliability of the final capacitor. This processing is complicated and consequently expensive.
After the sintered monolith is sintered connections to the inner electrodes must be formed by applying termination materials, typically thick film pastes, followed by additional sintering to contact the inner electrodes of the MLCC. This sintering step may be detrimental to the ceramic and/or internal conductors.
There are two metals systems typically used to create the internal electrodes of multi-layered ceramic capacitors. One system relies on precious metals such as silver, palladium, platinum or combinations thereof. Precious metals are advantageous since they can be used with air sinterable ceramics which is a significant manufacturing convenience. Unfortunately, precious metals are expensive and the price is highly volatile which leads to cost fluctuations in manufacturing. Base metal internal electrodes, such as nickel, have the advantage of lower material cost yet they are easily oxidized at sintering temperatures and therefore the ceramic must be fired in a reducing atmosphere, such as forming gas or nitrogen. As would be easily realized mixed metal systems can only be contemplated for a specific range of metals and alloys formulated to be compatible with ceramics due to these firing complexities.
There has been a long felt need for a method of forming capacitors, and of forming functionalized capacitors, using ceramic dielectrics which are sintered to achieve high density, without detrimental to the conductive layer or to the functionalized layer or terminations. This has previously been considered impossible.